Clinicians and researchers have long recognized that chronic insomnia arises from varied causes including primary sleep disorders, medical diseases, psychiatric illnesses, medication/substance abuse, and a host of behavioral and environmental factors. This recognition, in turn, has highlighted the need for a classification system to aid communication, structure diagnostic practice, and ultimately guide treatment decision-making among those who care for insomnia patients. Unfortunately, notable controversies over the nature and number of insomnia subtypes needing definition and the methods whereby insomnia diagnoses should be established have led to the emergence of two distinctive insomnia nosologies. One system, presented in the DSM-IV, postulates a limited number of global insomnia subtypes and advocates reliance on the clinical interview for diagnostic ascertainment. The other, presented in the International Classification of Sleep Disorders (ICSD), delineates many more highly specific insomnia subtypes and encourages consideration of sleep laboratory data in deriving diagnoses. Despite each of these system's fervent supporters, large prospective studies to test the reliability and validity of these nosologies and the diagnostic methods they encourage have yet to be conducted. The project proposed entails a prospective, dual-site investigation with a sizable (N = 448) research cohort designed to: (1) ascertain and compare the reliability and validity of the subtypes these two nosologies delineate; and (2) evaluate the contribution of sleep lab (polysomnographic - PSG) findings on the pattern of diagnoses assigned. The primary research methodology will consist of a multitrait-multimethod strategy in which three experienced clinician dyads derive DSM-IV and ICSD insomnia diagnostic impressions either through structured interviews, unstructured clinical interviews, or unstructured interviews combined with PSG findings. A resulting multitrait-multimethod matrix will be established for each nosology to evaluate the reliability, convergent validity, and discriminant validity of its various insomnia subtypes. In addition, all participants will complete a comprehensive psychometric battery and structured psychiatric interview (SCID) shortly after study enrollment to assess the concurrent validity of the DSM-IV and ICSD diagnostic subtypes. This project will provide a much-needed comparison of these two insomnia nosologies and the subtypes they define. Results also likely will provide important information for future insomnia classification efforts.